Downhole cutting tools having rolling cutters with non-planar cutting surfaces

ABSTRACT

A cutting tool may include a tool body having a plurality of blades extending radially therefrom; each of the plurality of blades having a plurality of cutter pockets formed therein; and at least one rotatable cutting element having a cutting table with a convex cutting surface mounted in at least one of the plurality of cutter pockets, wherein the at least one rotatable cutting element is mounted in a nose or shoulder region of the at least one of the plurality of blades.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Patent Provisional Application No. 61/865,859, filed on Aug. 14, 2013, which is herein incorporated by reference in its entirety.

BACKGROUND

Drill bits used to drill wellbores through earth formations generally are made within one of two broad categories of bit structures. Drill bits in the first category are generally known as “roller cone” bits, which include a bit body having one or more roller cones rotatably mounted to the bit body. The bit body is generally formed from steel or another high strength material. The roller cones are also generally formed from steel or other high strength material and include a plurality of cutting elements disposed at selected positions about the cones. The cutting elements may be formed from the same base material as is the cone. These bits are generally referred to as “milled tooth” bits. Other roller cone bits include “insert” cutting elements that are press (interference) fit into holes formed and/or machined into the roller cones. The inserts may be formed from, for example, tungsten carbide, natural or synthetic diamond, boron nitride, or any one or combination of hard or superhard materials.

Drill bits of the second category are generally referred to as “fixed cutter” or “drag” bits. This category of bits has no moving elements but rather have a bit body formed from steel or another high strength material and cutters (sometimes referred to as cutter elements, cutting elements or inserts) attached at selected positions to the bit body. For example, the cutters may be formed having a substrate or support stud made of carbide, for example tungsten carbide, and an ultra hard cutting surface layer or “table” made of a polycrystalline diamond material or a polycrystalline boron nitride material deposited onto or otherwise bonded to the substrate at an interface surface.

An example of a conventional drag bit having a plurality of cutters with ultra hard working surfaces is shown in FIG. 1 a. A drill bit 10 includes a bit body 12 and a plurality of blades 14 that are formed on the bit body 12. The blades 14 are separated by channels or gaps 16 that enable drilling fluid to flow between and both clean and cool the blades 14 and cutters 18. Cutters 18 are held in the blades 14 at predetermined angular orientations and radial locations to present working surfaces 20 with a desired backrake angle against a formation to be drilled. generally, the working surfaces 20 are generally perpendicular to the axis 19 and side surface 21 of a cylindrical cutter 18. Thus, the working surface 20 and the side surface 21 meet or intersect to form a circumferential cutting edge 22.

Nozzles 23 are often formed in the drill bit body 12 and positioned in the gaps 16 so that fluid can be pumped to discharge drilling fluid in selected directions and at selected rates of flow between the cutting blades 14 for lubricating and cooling the drill bit 10, the blades 14, and the cutters 18. The drilling fluid also cleans and removes the cuttings as the drill bit rotates and penetrates the geological formation. The gaps 16, which may be referred to as “fluid courses,” are positioned to provide additional flow channels for drilling fluid and to provide a passage for formation cuttings to travel past the drill bit 10 toward the surface of a wellbore (not shown).

The drill bit 10 includes a shank 24 and a crown 26. Shank 24 is generally formed of steel or a matrix material and includes a threaded pin 28 for attachment to a drill string. Crown 26 has a cutting face 30 and outer side surface 32. The particular materials used to form drill bit bodies are selected to provide adequate toughness, while providing good resistance to abrasive and erosive wear. For example, in the case where an ultra hard cutter is to be used, the bit body 12 may be made from powdered tungsten carbide (WC) infiltrated with a binder alloy within a suitable mold form. In one manufacturing process the crown 26 includes a plurality of holes or pockets 34 that are sized and shaped to receive a corresponding plurality of cutters 18.

The combined plurality of surfaces 20 of the cutters 18 effectively forms the cutting face of the drill bit 10. Once the crown 26 is formed, the cutters 18 are positioned in the pockets 34 and affixed by any suitable method, such as brazing, provides the pockets 34 inclined with respect to the surface of the crown 26. The pockets 34 are inclined such that cutters 18 are oriented with the working face 20 at a desired rake angle in the direction of rotation of the bit 10, so as to enhance cutting. It should be understood that in another construction (not shown), the cutters may each be substantially perpendicular to the surface of the crown, while an ultra hard surface is affixed to a substrate at an angle on a cutter body or a stud so that a desired rake angle is achieved at the working surface.

An example cutter 18 is shown in FIG. 1 b. The conventional cutter 18 has a cylindrical cemented carbide substrate body 38 having an end face or upper surface 54 referred to herein as the “interface surface” 54. An ultra hard material layer (cutting layer) 44, such as polycrystalline diamond or polycrystalline cubic boron nitride layer, forms the working surface 20 and the cutting edge 22. A bottom surface 52 of the ultra hard material layer 44 is bonded on to the upper surface 54 of the substrate 38. The bottom surface 52 and the upper surface 54 are herein collectively referred to as the interface 46. The top exposed surface or working surface 20 of the cutting layer 44 is opposite the bottom surface 52. The cutting layer 44 conventionally has a flat or planar working surface 20, but may also have a curved exposed surface, that meets the side surface 21 at a cutting edge 22.

Generally speaking, the process for making a cutter 18 employs a body of tungsten carbide as the substrate 38. The carbide body is placed adjacent to a layer of ultra hard material particles such as diamond or cubic boron nitride particles and the combination is subjected to high temperature at a pressure where the ultra hard material particles are thermodynamically stable. This results in recrystallization and formation of a polycrystalline ultra hard material layer, such as a polycrystalline diamond or polycrystalline cubic boron nitride layer, directly onto the upper surface 54 of the cemented tungsten carbide substrate 38.

One type of ultra hard working surface 20 for fixed cutter drill bits is formed as described above with polycrystalline diamond on the substrate of tungsten carbide, often known as a polycrystalline diamond compact (PDC), PDC cutters, PDC cutting elements, or PDC inserts. Drill bits made using such PDC cutters 18 are known generally as PDC bits. While the cutter or cutter insert 18 is often formed using a cylindrical tungsten carbide “blank” or substrate 38 which is sufficiently long to act as a mounting stud 40, the substrate 38 may also be an intermediate layer bonded at another interface to another metallic mounting stud 40.

The ultra hard working surface 20 is formed of the polycrystalline diamond material, in the form of a cutting layer 44 (sometimes referred to as a “table”) bonded to the substrate 38 at an interface 46. The top of the ultra hard layer 44 provides a working surface 20 and the bottom of the ultra hard layer cutting layer 44 is affixed to the tungsten carbide substrate 38 at the interface 46. The substrate 38 or stud 40 is brazed or otherwise bonded in a selected position on the crown of the drill bit body 12 (FIG. 1 a). As discussed above with reference to FIG. 1 a, the PDC cutters 18 are conventionally held and brazed into pockets 34 formed in the drill bit body at predetermined positions for the purpose of receiving the cutters 18 and presenting them to the geological formation at a rake angle.

Bits 10 using conventional PDC cutters 18 are sometimes unable to sustain a sufficiently low wear rate at the cutter temperatures generally encountered while drilling in abrasive and hard rock. These temperatures may affect the life of the bit 10, especially when the temperatures reach 700-750° C., resulting in structural failure of the ultra hard layer 44 or PDC cutting layer. A PDC cutting layer includes individual diamond “crystals” that are interconnected. The individual diamond crystals thus form a lattice structure. A metal catalyst, such as cobalt may be used to promote recrystallization of the diamond particles and formation of the lattice structure. Thus, cobalt particles are often found within the interstitial spaces in the diamond lattice structure. Cobalt has a different coefficient of thermal expansion as compared to diamond. Therefore, upon heating of a diamond table, the cobalt and the diamond lattice will expand at different rates, causing cracks to form in the lattice structure and resulting in deterioration of the diamond table.

It has been found by applicants that many cutters 18 develop cracking, spalling, chipping and partial fracturing of the ultra hard material cutting layer 44 at a region of cutting layer subjected to the highest loading during drilling. This region is referred to herein as the “cutting region” 56. The cutting region 56 encompasses the portion of the ultra hard material layer 44 that makes contact with the earth formations during drilling. The cutting region 56 is subjected to high magnitude stresses from dynamic normal loading, and shear loadings imposed on the ultra hard material layer 44 during drilling. Because the cutters are generally inserted into a drag bit at a rake angle, the cutting region 56 includes a portion of the ultra hard material layer near and including a portion of the layer's circumferential edge 22 that makes contact with the earth formations during drilling.

The high magnitude stresses at the cutting region 56 alone or in combination with other factors, such as residual thermal stresses, can result in the initiation and growth of cracks 58 across the ultra hard layer 44 of the cutter 18. Cracks of sufficient length may cause the separation of a sufficiently large piece of ultra hard material, rendering the cutter 18 ineffective or resulting in the failure of the cutter 18. When this happens, drilling operations may have to be ceased to allow for recovery of the drag bit and replacement of the ineffective or failed cutter. The high stresses, particularly shear stresses, may also result in delamination of the ultra hard layer 44 at the interface 46.

In some drag bits, PDC cutters 18 are fixed onto the surface of the bit 10 such that a common cutting surface contacts the formation during drilling. Over time and/or when drilling certain hard but not necessarily highly abrasive rock formations, the edge 22 of the working surface 20 that constantly contacts the formation begins to wear down, forming a local wear flat, or an area worn disproportionately to the remainder of the cutting element. Local wear flats may result in longer drilling times due to a reduced ability of the drill bit to effectively penetrate the work material and a loss of rate of penetration caused by dulling of edge of the cutting element. That is, the worn PDC cutter acts as a friction bearing surface that generates heat, which accelerates the wear of the PDC cutter and slows the penetration rate of the drill. Such flat surfaces effectively stop or severely reduce the rate of formation cutting because the conventional PDC cutters are not able to adequately engage and efficiently remove the formation material from the area of contact. Additionally, the cutters are generally under constant thermal and mechanical load. As a result, heat builds up along the cutting surface, and results in cutting element fracture. When a cutting element breaks, the drilling operation may sustain a loss of rate of penetration, and additional damage to other cutting elements, should the broken cutting element contact a second cutting element.

Additionally, another factor in determining the longevity of PDC cutters is the generation of heat at the cutter contact point, specifically at the exposed part of the PDC layer caused by friction between the PCD and the work material. This heat causes thermal damage to the PCD in the form of cracks which lead to spalling of the polycrystalline diamond layer, delamination between the polycrystalline diamond and substrate, and back conversion of the diamond to graphite causing rapid abrasive wear. The thermal operating range of conventional PDC cutters is generally 750° C. or less.

SUMMARY

This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

In one aspect, embodiments disclosed herein relate to a cutting tool that includes a tool body having a plurality of blades extending radially therefrom; each of the plurality of blades having a plurality of cutter pockets formed therein; and at least one rotatable cutting element having a cutting table with a convex cutting surface mounted in at least one of the plurality of cutter pockets, wherein the at least one rotatable cutting element is mounted in a nose or shoulder region of the at least one of the plurality of blades.

In another aspect, embodiments disclosed herein relate to a cutting tool that includes a tool body having a plurality of blades extending radially therefrom; each of the plurality of blades having a plurality of cutter pockets formed therein; and at least one rotatable cutting element having a cutting table with a concave cutting surface mounted in at least one of the plurality of cutter pockets, wherein the at least one rotatable cutting element is mounted in the shoulder of the at least one of the plurality of blades.

In yet another aspect, embodiments disclosed herein relate to a cutting tool that includes a tool body having a plurality of blades extending radially therefrom; and a plurality of rotatable cutting elements, wherein the plurality of rotatable cutting elements have differing cutting surface geometries based on their radial position along the plurality of blades.

Other aspects and advantages of the claimed subject matter will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A shows a perspective view of a conventional fixed cutter bit.

FIG. 1B shows a perspective view of a conventional PDC cutter.

FIGS. 2 and 3 show an embodiment of a rolling cutter with a convex cutting surface.

FIG. 4 illustrates the characteristic dimensions of a cutting element with a convex cutting surface.

FIGS. 5 and 6 show an embodiment of a rolling cutter with a concave cutting surface.

FIG. 7 shows an embodiment of a rolling cutter with a cutting surface having a concave region and a convex region.

FIG. 8 shows a profile of a bit as it would appear with each blade and each cutting elements rotated into a single rotated profile.

FIG. 9 shows a cutting structure profile of a bit according to embodiments disclosed herein.

FIG. 10 shows a comparison between the effective back rake angle of a cutting element having a convex cutting surface versus a planar cutting surface.

FIG. 11 shows the change in effective back rake angle as the cutting depth of a cutting element having a convex cutting surface is adjusted.

FIGS. 12 and 13 show profile views of a drill bit according to embodiments disclosed herein.

FIG. 14 shows an exploded view of a cutting element assembly according to embodiments of the present disclosure.

FIG. 15 shows a finite element analysis of a rotatable cutting element having a planar cutting face.

FIG. 16 shows a finite element analysis of a rotatable cutting element having a convex cutting surface.

DETAILED DESCRIPTION

In one or more aspects, embodiments disclosed herein relate to downhole tools (including fixed cutter drill bits) using rotatable cutting structures. In one or more aspects, embodiments disclosed herein relate to downhole tools (including fixed cutter drill bits) using rotatable cutting structures having non-planar cutting faces. Specifically, embodiments disclosed herein relate to improving the life of a drill bit (or other downhole tool) by positioning such rotatable cutting elements in particular arrangements on the drill bit.

Generally, rotatable cutting elements (also referred to as rolling cutters) described herein allow at least one surface or portion of the cutting element to rotate as the cutting elements contact a formation. As the cutting element contacts the formation, the cutting action may allow portion of the cutting element to rotate around a cutting element axis extending through the cutting element. Rotation of a portion of the cutting structure may allow for a cutting surface to cut the formation using the entire outer edge of the cutting surface, rather than the same section of the outer edge, as observed in a conventional cutting element. The following discussion describes various embodiments for a rotatable cutting element; however, the present disclosure is not so limited. One skilled in the art would appreciate that any cutting element capable of rotating may be used with the drill bit or other cutting tool of the present disclosure.

Non-Planar Cutting Surfaces

In addition to using rolling cutters, other strategies may be employed to enhance the utilization of rolling cutters. Specifically, in accordance with embodiments of the present disclosure, one or more of the rotatable cutting elements may have non-planar cutting surfaces. In more particular embodiments, the cutting surface of the rotatable cutting elements may be substantially convex or concave. FIGS. 2-3 and FIGS. 5-6 show representative embodiments of cutting elements with a substantially convex or concave cutting surface, respectively. In addition to use of non-planar cutting surfaces (and the extent of non-planarity), the present disclosure also relates to the placement of such non-planar cutting surfaces along a blade.

FIGS. 2 and 3 illustrate an embodiment of a rotatable cutting element. As shown in FIGS. 2-3, a rotatable cutting element 300 possesses a diamond (or other ultrahard material) body 305 disposed on a substrate 301. The diamond body 305 has convex upper or cutting surface 306 with an axial apex 307 extending a height from the plane (extending perpendicular to an axis of the cutting element 300) through cutting edge 304. Edge 304 is defined as the edge formed between a circumferential side surface 302 and cutting surface 306. Extending away from diamond table 305, substrate 301 includes a shaft portion 308, smaller in diameter than the portion of the substrate 301 interfacing the diamond table 305. Further, the incorporation of a shaft 308 may be particularly desirable for a rotatable cutting element that is assembled with a sleeve; however, the present disclosure is not so limited. Rather, rotatable cutting elements used without sleeves may also be used, and in such cases, the substrate may, but not necessarily, be designed without a shaft having a reduced diameter. Further, it is also within the scope of the present disclosure that a sleeve may be used without a shaft portion. Thus, the present disclosure does not limit the type of rolling cutter used, but instead is directed to the shape of the cutting surface, and placement of the cutting elements having such non-planar cutting surfaces on a downhole cutting tool.

Referring to FIG. 4, a convex cutting surface 306 is shown. Diamond table 305 has a base 303 forming the circumferential side surface with a diameter D. Diamond table 305 extends axially away from base 303 to convex cutting surface 306 terminating at an apex 307. Apex 307 is spaced a distance X from base (measured from the plane extending through cutting edge 304). Incorporation of a convex cutting surface may change the effective angle between the cutting element and formation (i.e. effective backrake) as the cutting element cuts into the formation. Specifically, such angle may change depending on the depth of cut of the cutting element into the formation. Thus, the inventors of the present application have determined that a particular range of convexity results in an advantageous effect on the impact strength of the cutting element. The convexity may be considered as the ratio of X/D, i.e., the axial rise of the cutting surface 306 to its apex 307 relative to the size (diameter) of the diamond table 305. In one or more embodiments, the ratio of X/D may range from 0.015 to 0.21. In more specific embodiments, the ratios of x/d for the cutting elements with convex cutting surfaces may range from 0.02 to 0.1, or from 0.025 to 0.065. In one or more other embodiments, the ratio of X/D may have a lower limit of any of 0.015, 0.017, 0.020, 0.022, 0.025, 0.026, 0.03, or 0.04 and an upper limit of any of 0.21, 0.20, 0.15, 0.10, 0.075, 0.065, 0.06, 0.055 or 0.05 where any lower limit can be used with any upper limit. The extension may reduce the stress within the cutting element when the cutting element is subjected to a frontal load. However, too much convexity results in higher thermal residual stresses. The inventors of the present application have found that a simultaneous reduction in stress under frontal loading as well as thermal residual stress may be achieved when the convexity is within the above described ranges for the X/D ratio. Such reduction in stress may result in a cutting element that is better suited for high impact applications as compared to a non-planar cutting element or other shapes falling outside of this ratio range.

The values for the diameter D of the base 303 of the diamond table 305 may be in the range from 0.5 inches to 0.65 inches. The values for the distance X, or extension height of the cutting table, may be in the range from 0.005 inches to 0.25 inches. One skilled in the art would appreciate that depending on the desired convexity (and the size of cutting element desired for the tool design), an appropriate extension height of the cutting table may be selected. Convexity can also be considered as radius of curvature; however, like the extension values, radius of curvature also does not consider the size of the cutting element. The radius of curvature of the cutting element may range, for example, from 0.375 to 4 inches, or from 1.0 to 3.8 inches. Further, in particular embodiments, the convex cutting surface 306 may have a substantially constant radius of curvature along the entire cutting surface 306 from cutting edge 304 to apex 307.

The present disclosure also relates to the use of rotatable cutting elements having concave cutting surfaces. Referring now to FIGS. 5 and 6, another embodiment of a rotatable cutting element having a non-planar cutting surface is shown. As shown in FIGS. 5 and 6, a rotatable cutting element 500 includes a diamond (or other ultrahard material) table 505 disposed on a substrate 501. The diamond body 505 has concave upper or cutting surface 506 with an axial extremity (lowest point) 507 extending below the plane through cutting edge 504. Thus, concave cutting surface 506 results in a decreasing diamond table 505 thickness extending inwardly from the outer radial or cutting edge 504 to the extremity 507, which may (or may not be) near the radial center of the cutting element. Extending away from diamond table 505, substrate 501 may include a shaft portion 508, smaller in diameter than the portion of the substrate 501 interfacing the diamond table 505. Further, the incorporation of a shaft 508 may be particularly desirable for a rotatable cutting element that is assembled with a sleeve; however, the present disclosure is not so limited. Rather, rotatable cutting elements used without sleeves may also be used, and in such cases, the substrate may, but not necessarily, be designed without a shaft having a reduced diameter. Further, it is also within the scope of the present disclosure that a sleeve may be used without a shaft portion. In a more particular embodiment, the rotatable cutting element with a substantially concave cutting surface possesses a substantially uniform radius of curvature along the concave cutting surface 506 from cutting edge 504 to extremity 507. Advantageously, the concave cutting surface may result in a sharper cutting edge, which may provide for better cutting efficiency compared to a planar cutting surface. The ratio of the depth of the concavity to the diameter of the cutter may range from 0.01 to 0.25, with a lower of any of 0.01, 0.025, 0.05, and 0.1 and an upper limit of any of 0.1, 0.125, 0.15, 0.2, 0.225, and 0.25, where any lower limit can be used in combination with any upper limit.

Further, the present disclosure also may incorporate a rotatable cutting element having a variable non-planar cutting surface, having both convex and concave portions. Referring now to FIG. 7, an embodiment of such a cutting element is sown. As shown, cutting surface 706 of rotatable cutting element 700 may have a concave region 762 extending radially inward from a cutting edge 704 a selected distance which transitions to a convex region 764 extending radially inward from concave region 762 to the axis of the cutting element 700. In one or more embodiments, the concave region 762 may extend a distance of about 0.005 inches to 0.3 inches radially inward from cutting edge 704 before transitioning to a convex region 764. Selection of the inward extension distance of the concave region 762 before transitioning to a convex region 764 may be based upon the desired cutting efficiency as well depth of cut control (the transition to the convex region 764 may dictate the depth of cut into the formation). In one or more embodiments, the apex 707 of the convex region 764 may extend above the axial extent of the cutting edge 704. However, in one or more embodiments the apex 707 of the convex region 764 may be at substantially the same axial height as the cutting edge 704 or may be at a lower axial height relative to the cutting edge 704.

Placement of Rolling Cutters

According to embodiments of the present disclosure, a bit design consideration may include placement of rolling cutters on a drill bit. Placement design of rolling cutters on a drill bit may involve, first, predicting where conventional cutter (fixed cutter) impact damage occurs most frequently or quickly on a drill bit. For example, fixed cutter wear may be predicted using engineering and design software, such as I-DEAS, “Integrated Design and Engineering Analysis Software”, or CAD software. Such engineering and design software may also be used to optimize bit stabilization dynamics using various placements of rolling cutters. Fixed cutter impact damage may also be predicted by observing and/or measuring impact damage on dull drill bits. In particular, as a drill bit having conventional, fixed cutters contacts and cuts an earthen formation, the cutting surface and cutting edge of a fixed cutter may wear and form a wear flat.

Once fixed cutter impact damage is predicted, criteria for the placement of rolling cutters having a non-planar cutting surface may be set according to where the impact damage occurs. For example, according to embodiments of the present disclosure, rolling cutter placement design may include placing rolling cutters having a convex cutting surface in a place where impact damage and wear occurs. Further, in other embodiments, rolling cutter placement design may include replacing cutters (rolling or not) having planar cutting surfaces with rolling cutters having non-planar cutting surfaces on certain blades of a drill bit.

In some embodiments of the present disclosure, rolling cutter placement design criteria may be set so that rolling cutters are positioned in the areas of the bit experiencing the greatest wear. For example, rolling cutters may be placed in the shoulder region of a drill bit. Referring to FIG. 8, a profile 39 of a bit 10 is shown as it would appear with each blade and each cutting element (including primary cutting elements and back up cutting elements) rotated into a single rotated profile. A blade profile 39 (most clearly shown in the right half of bit 10 in FIG. 8) may generally be divided into three regions conventionally labeled cone region 24, shoulder region 25, and gage region 26. Cone region 24 comprises the radially innermost region of bit 10 (e.g., cone region 24 is the central most region of bit 10) and composite blade profile 39 extending generally from bit axis 11 to shoulder region 25. As shown in FIG. 8, in most fixed cutter bits, cone region 24 is generally a concave portion of the blade. Adjacent cone region 24 is shoulder (or the upturned curve) region 25. Thus, composite blade profile 39 of bit 10 includes one concave region of the blade—cone region 24, and one convex region of the blade—shoulder region 25. In most fixed cutter bits, shoulder region 25 is generally the convex region of the blade. Moving radially outward, adjacent shoulder region 25 is the gage region 26 which extends parallel to bit axis 11 at the outer radial periphery 23 of composite blade profile 39. Outer radius 23 extends to and therefore defines the full gage diameter of bit 10. Cone region 24 is defined by a radial distance along the x-axis measured from central axis 11. It is understood that the x-axis is perpendicular to central axis 11 and extends radially outward from central axis 11. Cone region 24 may be defined by a percentage of outer radius 23 of bit 10. The actual radius of cone region 24, measured from central axis 11, may vary from bit to bit depending on a variety of factors including without limitation, bit geometry, bit type, location of one or more secondary blades, location of back up cutting elements 50, or combinations thereof. The axially lowermost point of the convex shoulder region 25 and composite blade profile 39 defines a blade profile nose 27. At blade profile nose 27, the slope of a tangent line 27 a to convex shoulder region 25 and composite blade profile 39 is zero. Thus, as used herein, the term “blade profile nose” refers to the point along a convex region of a composite blade profile of a bit in rotated profile view at which the slope of a tangent to the composite blade profile is zero. For most fixed cutter bits (e.g., bit 10), the composite blade profile includes a single convex shoulder region on a blade (e.g., convex shoulder region 25), and a single blade profile nose (e.g., nose 27). Advantageously, by placing rolling cutters with non-planar cutting surfaces in areas of the bit experiencing the greatest wear and load, for example at the shoulder region 26 and nose region 27 of a bit, the wear rate of the bit may be improved.

As discussed above, in one or more embodiments, depending on the positioning of the rotatable cutting element along the blade, the cutting surface geometry may be selected. While the non-planar cutting surface roller cutters may be used in any location, in one or more particular embodiments, at least one rotatable cutting element with a convex cutting surface may be located in the cone and/or nose of at least one of the blades on the bit. Advantageously, the convex cutting surface geometry presents a rotatable cutter with a higher impact resistance thereby making it suitable for placement in the nose of the bit where the load is higher on the cutters therein and greater impact resistance is desirable. Further, in one or more embodiments, at least one rotatable cutting element with concave cutting surface may be located in the shoulder of at least one of the blades on the bit. Such cutting elements located in the shoulder may benefit from a greater cutting efficiency based on the greater scraping distance (per bit revolution) experienced by those cutters, as compared to more radially inward cutting elements. It is also within the scope of the present disclosure that rotatable cutters with a any type of non-planar cutting surface may be used in combination with rotatable cutters having a planar surface and/or conventional non-rotatable cutters. For example, in one or more embodiments, rotatable cutting elements with a convex surface may be used in the nose region, while non-rotatable cutting elements are used in the cone and gage regions. In such embodiments, the cutters in the shoulder region may be rotatable and have any type of cutting surface.

Additionally, in one or more embodiments, rotatable cutting elements with a concave surface may be used in shoulder nose region, while non-rotatable cutting elements are used in the cone and gage regions. In such embodiments, the cutters in the nose region may be rotatable and have any type of cutting surface.

When placed on a blade, rotatable cutting elements (as well as fixed cutters) may be oriented with a back rake and/or side rake. Referring to FIG. 9, a cutting structure profile of a bit is shown to aid in the understanding of the side rake and back rake of cutting elements. As shown, cutters 2600 positioned on a blade 2602 may have side rake or back rake. Back rake is generally defined as the angle subtended between the cutting face of the cutter 2600 and a line parallel to the longitudinal axis 2607 of the bit. However, for a non-planar cutting surface, the angle would upon where along the cutting surface from which the angle is being taken. Thus, for non-planar cutting elements, the plane extending through the cutting edge (discussed above with respect to the measure of convexity or concavity) may be used to determine the orientation of the cutting element on the blade. Side rake is generally defined as the angle between the cutting face and the radial plane of the bit (x-z plane). For a non-planar cutting surface, the side rake may be defined based on the plane 2605 extending through the cutting edge (as discussed above). When viewed along the z-axis, a negative side rake results from counterclockwise rotation of the cutter 2600, and a positive side rake, from clockwise rotation.

In some embodiments, a planar cutter may have a back rake ranging from about 5 to 35 degrees. In a particular embodiment, the back rake angle of a rolling and/or fixed cutter with a planar cutting surface may be >5 degrees, >10 degrees, >15 degrees, >20 degrees, >25 degrees, >30 degrees, and/or <10 degrees, <15 degrees, <20 degrees, <25, <30 degrees, <35 degrees, with any upper limit being used with any lower limit. Such back rake angles may be used for rolling and/or fixed cutters in any of the cone, nose, shoulder or gage region of the bit, but in particular embodiments, a back rake of between 10 and 35 degrees (or 15 to 35 degrees or 20 to 30 degrees in more particular embodiments) may be particularly suitable for rolling cutters in the nose and/or shoulder region of the bit. A cutter may be positioned on a blade with a selected back rake to assist in removing drill cuttings and increasing rate of penetration.

A cutter disposed on a drill bit with side rake may be forced forward in a radial and tangential direction when the bit rotates. In some embodiments, because the radial direction may assist the movement of a rotatable cutting element, such rotation may allow greater drill cuttings removal and provide an improved rate of penetration. In one embodiment, a cutter may have a side rake ranging from 0 to ±45 degrees, for example ±5 to ±35 degrees, ±10 to ±35 degrees or ±15 to ±30 degrees. In a particular embodiment, the direction (positive or negative) of the side rake may be selected based on the cutter distribution, i.e., whether the cutters are arranged in a forward or reverse spiral configuration. For example, in embodiments, if cutters are arranged in a reverse spiral, positive side rake angles may be particularly desirable. Conversely, if cutters are arranged in a forward spiral, negative side rake angles may be particularly desirable.

One of ordinary skill in the art may realize that any back rake and side rake combination may be used with the cutting elements of the present disclosure to enhance rotatability and/or improve drilling efficiency. In one or more other embodiments, cutting elements may be disposed in cutting tools that do not incorporate back rake and/or side rake. When the cutting element is disposed on a drill bit with substantially zero degrees of side rake and/or back rake, the cutting force may be random instead of pointing in one general direction. The random forces may cause the cutting element to have a discontinuous rotating motion. Generally, such a discontinuous motion may not provide the most efficient drilling condition, however, in certain embodiments, it may be beneficial to allow substantially the entire cutting surface of the insert to contact the formation in a relatively even manner. In such an embodiment, other inner rotatable cutting element and/or cutting surface designs may be used to further exploit the benefits of rotatable cutting elements. Further, in one or more other embodiments, a bevel or chamfer size, angle, or design may be selected to accommodate for a zero back or side rake.

As mentioned above, use of a non-planar cutting surface changes the angle that the cutting surface makes with respect to the formation (and changing the effective back rake angle of the cutter from the perspective of the formation). Moreover, because of the non-planarity, the angle also may also change depending on the depth of cut of the cutting element into the formation. Referring to FIG. 10, the depiction of the change in effective back rake angle for a convex cutting surface is shown. As shown in FIG. 10, a back rake of angle β is formed between a line normal to the formation 172 and the plane 174 extending through the cutting edge 304 (which would be the back rake of a cutting element with a planar cutting surface). However, angle α is defined between the line normal to the formation 172 and the tangent 176 of convex cutting surface 306 at the cutting edge 304, and angle α is clearly greater than angle β.

As also mentioned above, the effective back rake may also change depending on the depth of cut into the formation. Referring now to FIG. 11, such change is illustrated. Specifically, as illustrated in FIG. 11, the impact on the effective back rake angle with respect to the formation is dependent on the depth of cut experienced by the cutting element. Specifically, as the cutting element digs deeper into the formation, the leading point of the cutting element that interfaces the formation will dictate the effective back rake angle. Thus, as shown in FIG. 11, for a cutting element with a convex cutting surface 306, as the depth of cut (DOC) 602 increases from 0.05 to 0.10 to 0.15 to 0.2645 inches, the observed effective back rake angle (defined as being the angle between the line parallel to the longitudinal axis of the bit and a line between the cutting edge 304 and the leading point of the cutting element that interfaces the formation) correspondingly decreases. Such decreasing angle is shown as the angles formed between lines 601, 603, 605, and 607 with the line 606 parallel to the longitudinal axis of the bit. However, because the effective back rake angle with respect to the formation is dependent upon both cutter orientation, non-planarity, and depth of cut, it may be more simplistic to refer to back rake based on cutter orientation and also contemplate a non-planar angle for the cutting element. Such non-planar angle may be defined as the angle formed between the plane extending through the cutting edge 304 and the tangent to the convex cutting surface 306 at the cutting edge 304. In one or more embodiments, such non-planar angle may be at least 2, 3, 4 or 5 degrees and less than 10, 9, 8, 7, 6, or 5 degrees, where any lower limit can be used in combination with any upper limit.

According to embodiments of the present disclosure, rolling cutter placement design criteria may be set so that rolling cutters and fixed cutters on a drill bit have a plural set configuration. Drill bits having a plural set configuration have more than one cutting element at at least one radial position with respect to the bit axis. Expressed in another way, at least one cutting element includes a “back up” cutting element disposed at about the same radial position with respect to the bit axis. For example, referring to FIGS. 12 and 13, a face side profile view of a drill bit 2400 having a plurality of cutting blades 2410 are shown, wherein the bits rotate in direction R. Primary blades 2410 a extend radially from substantially proximal the longitudinal axis A of the bit toward the periphery of the bit. Secondary blades 2410 b do not extend from substantially proximal the bit axis A, but instead extend radially from a location that is a distance away from the bit axis A. Cutting elements 2420, 2430 are positioned at the leading side of blades 2410, wherein the leading sides of blades 2410 face in the direction of bit rotation R and trailing sides of blades face the opposite direction. Further, as shown, cutting element 2420 trails cutting element 2430 in plural set configuration, i.e., cutting element 2420 “backs up” cutting element 2430 at about the same radial position with respect to the bit axis A. Either cutting element 2420 or cutting element 2430, or both cutting elements 2420 and 2430, may be rolling cutters. In a particular embodiment, a bit having a plural set cutter configuration may have at least one trailing or backup cutting element that is rotatable (a rolling cutter) and at least one leading or primary cutting element that is a fixed cutter. In another embodiment, a bit having a plural set configuration may have at least one fixed cutter trailing cutting element and at least one rolling cutter leading cutting element. Advantageously, by using a plural set configuration having at least one rolling cutter, the cutting structure may be more robust.

Further, a bit may have a single set configuration of cutting elements, wherein each cutting element in a single set configuration is at a unique radial position of the bit. In embodiments having a single set configuration, a plurality of rolling cutters may be placed at various unique radial positions with respect to the bit axis. For example, a plurality of rolling cutters may have a forward spiral or a reverse spiral single set configuration, wherein the rolling cutters are placed in areas experiencing wear. As used herein, a forward spiral layout refers to a cutter placement where cutters having incrementally increasing radial distances from the bit centerline are placed in a clockwise distribution whereas a reverse spiral layout refers to a cutter placement where cutters having incrementally increasing radial distances from a bit centerline are placed in a counterclockwise distribution. In some embodiments, the cutters may be placed in a forward spiral, where rotatable cutters are at least placed in the nose and/or shoulder region, are placed in the nose, shoulder, and gage regions in particular embodiments, and are placed in the cone, nose, shoulder, and gage regions in more particular embodiments. In some embodiments, the cutters may be placed in a reverse spiral, where rotatable cutters are at least placed in the nose and/or shoulder region, are placed in the nose, shoulder, and gage regions in particular embodiments, and are placed in the cone, nose, shoulder, and gage regions in more particular embodiments.

Additionally, leading and trailing cutting elements may be placed on a single blade. However, as used herein, the term “backup cutting element” is used to describe a cutting element that trails any other cutting element on the same blade when the bit is rotated in the cutting direction. Further, as used herein, the term “primary cutting element” is used to describe a cutting element provided on the leading edge of a blade. In other words, when a bit is rotated about its central longitudinal axis in the cutting direction, a “primary cutting element” does not trail any other cutting elements on the same blade. Suitably, each primary cutting elements and optional backup cutting element may have any suitable size and geometry. Primary cutting elements and backup cutting elements may have any suitable location and orientation and may be rolling cutters or fixed cutters. In an example embodiment, backup cutting elements may be located at the same radial position as the primary cutting element it trails, or backup cutting elements may be offset from the primary cutting element it trails, or combinations thereof may be used.

In particular, each blade on a bit face (e.g., primary blades and secondary blades) provides a cutter-supporting surface to which cutting elements are mounted. Primary cutting elements may be disposed on the cutter-supporting surface of the blades and one or more of the primary blades may also have backup cutting elements disposed on the cutter-supporting surface of the bit. In one or more embodiments, backup cutting elements may be provided on the cutter-supporting surface of one or more of the bit primary blades in the cone region. In a different example embodiment, backup cutting elements may be provided on the cutter-supporting surface of any one or more secondary blades in the shoulder and/or gage region. In another example embodiment, backup cutting elements may be provided on the cutter-supporting surface of any one or more primary blades in the gage region. In yet another example embodiment, the primary and/or secondary blades may have at least two rows of backup cutting elements disposed on the cutter-supporting surfaces.

Primary cutting elements may be placed adjacent one another generally in a first row extending radially along each primary blade of a bit and along each secondary blade of a bit. Further, backup cutting elements may be placed adjacent one another generally in a second row extending radially along each primary blade in the shoulder region. Suitably, the backup cutting elements form a second row that may extend along each primary blade in the shoulder region, cone region and/or gage region. Backup cutting elements may be placed behind the primary cutting elements on the same primary blade, wherein backup cutting elements trail the primary cutting elements on the same primary blades.

In general, primary cutting elements as well as backup cutting elements do not have to be positioned in rows, but may be mounted in other suitable arrangements provided each cutting element is either in a leading position (e.g., primary cutting element) or a trailing position (e.g., backup cutting element). Examples of suitable arrangements may include without limitation, rows, arrays or organized patterns, randomly, sinusoidal pattern, or combinations thereof. Further, in other embodiments, additional rows of cutting elements may be provided on a primary blade, secondary blade, or combinations thereof.

Further, in a particular embodiment, a bit may have cutting elements placed in a single set configuration with rolling cutters with non-planar cutting surfaces placed in areas of the bit experiencing the greatest wear. In another embodiment, a bit may have cutting elements placed in a plural set configuration, wherein at least one rolling cutter with a non-planar cutting surface is placed in areas of the bit experiencing the greatest wear.

Embodiments of Rolling Cutters

Rolling cutters of the present disclosure may include various types and sizes of rolling cutters. For example, rolling cutters may be formed in sizes including, but not limited to, 9 mm, 13 mm, 16 mm, and 19 mm. Further, the type of rolling cutter is of no limitation to the present disclosure. Rather, it may be of any type and/or include any feature such as those described in U.S. Pat. No. 7,703,559, U.S. Patent Publication Nos. 2011/0297454, 2012/0273280, 2012/0273281, 2012/0273281, and 2014/0054094, all of which are assigned to the present assignee and herein incorporated by reference in their entirety. Embodiments of rolling cutters are also described below; however, the types of rotatable cutting elements that may be used with the present disclosure are not necessarily limited to those described below.

Referring now to FIG. 14, a rotatable cutting element assembly according to embodiments of the present disclosure is shown. Particularly, an exploded view of the cutting element is shown in FIG. 14, including a rolling cutter 300, a retaining ring 320, and a sleeve 330. The rolling cutter 300 has an axis of rotation extending longitudinally therethrough, a cutting face 306, and a substrate 301 extending axially downward from the cutting face 306. The substrate 301 includes a shaft portion 308 with a reduced diameter as compared to the rest of the substrate 301. A circumferential groove 310 is formed on a shaft 308 portion of the substrate 301. Further, a cutting edge 304 is formed at the intersection of the cutting face 306 and the outer surface 302 of the rolling cutter 300. As shown, the cutting face 306 and cutting edge 304 may be formed from a diamond or other ultra-hard material table 305.

As assembled, the cutting element has a retaining ring 320 disposed in the circumferential groove 310, wherein the retaining ring 320 extends at least around the entire circumference of the shaft 308. For example, in the embodiment shown in FIG. 14, the retaining ring 320 may extend greater than 1.5 times around the circumference of the shaft 308. As shown in FIG. 14, the retaining ring 320 protrudes from the circumferential groove 310 to extend into an inner groove (not shown) of the sleeve 330, thereby retaining the rolling cutter 300 within the sleeve 330.

Further, in one or more embodiments, the rotatable cutting elements placed in the areas of the bit experiencing the greatest wear may have a thicker substrate table extending above the sleeve and supporting the cutting surface than cutting elements placed in areas which experience less wear. In more particular embodiments, the rotatable cutting elements placed in the nose region have a thicker substrate table extending above the sleeve and supporting the cutting surface than rotatable cutting elements located in the shoulder region.

However, the present disclosure is not limited to the rolling cutter type illustrated in FIG. 14, but instead, as mentioned above, any type of rolling cutter may be used on the bits and tools of the present disclosure.

In various embodiments, the cutting face of the inner rotatable cutting element may include an ultra hard layer that may be comprised of a polycrystalline diamond table, a thermally stable diamond layer (i.e., having a thermal stability greater than that of conventional polycrystalline diamond, 750° C.), or other ultra hard layer such as a cubic boron nitride layer.

As known in the art, thermally stable diamond may be formed in various manners. A conventional polycrystalline diamond layer includes individual diamond “crystals” that are interconnected. The individual diamond crystals thus form a lattice structure. A metal catalyst, such as cobalt, may be used to promote recrystallization of the diamond particles and formation of the lattice structure. Thus, cobalt particles are generally found within the interstitial spaces in the diamond lattice structure. Cobalt has a different coefficient of thermal expansion as compared to diamond. Therefore, upon heating of a diamond table, the cobalt and the diamond lattice will expand at different rates, causing cracks to form in the lattice structure and resulting in deterioration of the diamond table.

To obviate this problem, strong acids may be used to “leach” the cobalt from a polycrystalline diamond lattice structure (either a thin volume or entire tablet) to at least reduce the damage experienced from heating diamond-cobalt composite at different rates upon heating. Examples of “leaching” processes can be found, for example, in U.S. Pat. Nos. 4,288,248 and 4,104,344. Briefly, a strong acid, such as hydrofluoric acid or combinations of several strong acids may be used to treat the diamond table, removing at least a portion of the co-catalyst from the PDC composite. Suitable acids include nitric acid, hydrofluoric acid, hydrochloric acid, sulfuric acid, phosphoric acid, or perchloric acid, or combinations of these acids. In addition, caustics, such as sodium hydroxide and potassium hydroxide, have been used to the carbide industry to digest metallic elements from carbide composites. In addition, other acidic and basic leaching agents may be used as desired. Those having ordinary skill in the art will appreciate that the molarity of the leaching agent may be adjusted depending on the time desired to leach, concerns about hazards, etc.

By leaching out the cobalt, thermally stable polycrystalline (TSP) diamond may be formed. In certain embodiments, a select portion of a diamond composite is leached, in order to gain thermal stability without losing impact resistance. As used herein, the term TSP includes both of the above (i.e., partially and completely leached) compounds. Interstitial volumes remaining after leaching may be reduced by either furthering consolidation or by filling the volume with a secondary material, such by processes known in the art and described in U.S. Pat. No. 5,127,923, which is herein incorporated by reference in its entirety.

In another embodiment, TSP may be formed by forming the diamond layer in a press using a binder other than cobalt, one such as silicon, which has a coefficient of thermal expansion more similar to that of diamond than cobalt has. During the manufacturing process, a large portion, 80 to 100 volume percent, of the silicon reacts with the diamond lattice to form silicon carbide which also has a thermal expansion similar to diamond. Upon heating, any remaining silicon, silicon carbide, and the diamond lattice will expand at more similar rates as compared to rates of expansion for cobalt and diamond, resulting in a more thermally stable layer. PDC cutters having a TSP cutting layer have relatively low wear rates, even as cutter temperatures reach 1200° C. However, one of ordinary skill in the art would recognize that a thermally stable diamond layer may be formed by other methods known in the art, including, for example, by altering processing conditions in the formation of the diamond layer.

The substrate on which the cutting face is disposed may be formed of a variety of hard or ultra hard particles. In one embodiment, the substrate may be formed from a suitable material such as tungsten carbide, tantalum carbide, or titanium carbide. Additionally, various binding metals may be included in the substrate, such as cobalt, nickel, iron, metal alloys, or mixtures thereof. In the substrate, the metal carbide grains are supported within the metallic binder, such as cobalt. Additionally, the substrate may be formed of a sintered tungsten carbide composite structure. It is well known that various metal carbide compositions and binders may be used, in addition to tungsten carbide and cobalt. Thus, references to the use of tungsten carbide and cobalt are for illustrative purposes, and no limitation on the type substrate or binder used is intended. In another embodiment, the substrate may also be formed from a diamond ultra hard material such as polycrystalline diamond and thermally stable diamond. While the illustrated embodiments show the cutting face and substrate as two distinct pieces, one of skill in the art should appreciate that it is within the scope of the present disclosure the cutting face and substrate are integral, identical compositions. In such an embodiment, it may be desirable to have a single diamond composite forming the cutting face and substrate or distinct layers.

The cutting elements of the present disclosure may be incorporated in various types of downhole cutting tools, including for example, as cutters in fixed cutter bits or as inserts in roller cone bits, reamers, hole benders, or any other tool that may be used to drill earthen formations. Cutting tools having the cutting elements of the present disclosure may include a single rotatable cutting element with the remaining cutting elements being conventional cutting elements, each cutting element on the tool being rotatable, or any combination therebetween of rotatable and conventional cutting elements.

The cutting elements of the present disclosure may be attached to or mounted on a drill bit by a variety of mechanisms, including but not limited to conventional attachment or brazing techniques in a cutter pocket, as well as by mechanical means. It is also within the scope of the present disclosure that in some embodiments, an inner rotatable cutting element may be mounted on the bit directly such that the bit body acts as the outer support element, i.e., by inserting the inner rotatable cutting element into a hole that may be subsequently blocked to retain the inner rotatable cutting element within.

While the above describes a situation where the rolling cutters have a non-planar cutting surface, it is also contemplated that planar cutting surfaces may also be used on rolling cutters. In other words, it is expressly within the scope of the present disclosure that the rolling cutters may comprise mixtures of planar and non-planar cutting surfaces. Additionally, if each of the cutters are not rolling cutters, the fixed cutters may possess planar cutting surfaces, non-planar cutting surfaces, or mixtures thereof.

EXAMPLE

A finite element analysis (FEA) was performed on a rotatable cutting element having a planar cutting face (shown in FIG. 16) and a convex cutting surface (shown in FIG. 17). The FEA results showed that the cutting element with a planar cutting face has a maximum principal stress of 118.8 ksi whereas the cutting element with the convex cutting face has a maximum principal stress of 100.4 ksi, with the higher stress being present particularly at the transition region between the radial bearing surface and the shaft of the rotatable cutting element. This FEA analysis shows that the cutting element with the convex cutting surface shows an 18% better strength than the planar cutting face under frontal loads.

Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from this invention. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. §112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function. 

What is claimed:
 1. A cutting tool comprising: a tool body having a plurality of blades extending radially therefrom; each of the plurality of blades having a plurality of cutter pockets formed therein; and at least one rotatable cutting element having a cutting table with a convex cutting surface mounted in at least one of the plurality of cutter pockets, wherein the at least one rotatable cutting element is mounted in a nose or shoulder region of the at least one of the plurality of blades.
 2. The cutting tool of claim 1, wherein the convex cutting surface is a convex region extending radially from the center of the cutting element a distance, after which the cutting surface surrounding the convex region transitions to form a concave rim.
 3. The cutting tool of claim 1, wherein the convex cutting table has a ratio of extension at apex to cutting table base diameter ranging from 0.015 to 0.21.
 4. The cutting tool of claim 3, wherein the convex cutting table has a ratio of extension at apex to cutting table base diameter ranging from 0.025 to 0.065.
 5. The cutting tool of claim 1, further comprising a plurality of rotatable cutting elements mounted in the shoulder region of at least one of the plurality of blades.
 6. The cutting tool of claim 5, wherein the plurality of rotatable cutting elements mounted in the shoulder region have a cutting table with a planar and/or non-planar cutting surface disposed on a substrate, wherein a portion of the substrate has a reduced diameter that is disposed within a sleeve.
 7. The cutting tool of claim 5, wherein the plurality of rotatable cutting elements located in the nose region have a portion of the substrate extending above the sleeve that is greater than the portion of the substrate extending above the sleeve for rotatable cutting elements located in the shoulder region.
 8. The cutting tool of claim 1, further comprising a plurality of non-rotatable cutting elements mounted on at least one of the plurality of blades.
 9. A cutting tool comprising: a tool body having a plurality of blades extending radially therefrom; each of the plurality of blades having a plurality of cutter pockets formed therein; and at least one rotatable cutting element having a cutting table with a concave cutting surface mounted in at least one of the plurality of cutter pockets, wherein the at least one rotatable cutting element is mounted in the shoulder of the at least one of the plurality of blades.
 10. The cutting tool of claim 9, further comprising a plurality of rotatable cutting elements mounted in the nose region of at least one of the plurality of blades.
 11. The cutting tool of claim 10, wherein the plurality of rotatable cutting elements mounted in the nose region have a planar and/or non-planar cutting surface.
 12. The cutting tool of claim 10, wherein the plurality of rotatable cutting elements mounted in the nose region have a cutting table with a convex cutting surface disposed on a substrate, wherein a portion of the substrate has a reduced diameter that is disposed within a sleeve.
 13. The cutting tool of claim 12, wherein the cutting table with a convex cutting surface has a ratio of extension at apex to cutting table base diameter ranging from 0.015 to 0.21.
 14. The cutting tool of claim 13, wherein the cutting table with a convex cutting surface has a ratio of extension at apex to cutting table base diameter ranging from 0.025 to 0.065.
 15. The cutting tool of claim 10, wherein the plurality of rotatable cutting elements located in the nose region have a portion of the substrate extending above the sleeve that is greater than the portion of the substrate extending above the sleeve for the rotatable cutting elements located in the shoulder region.
 16. A cutting tool comprising: a tool body having a plurality of blades extending radially therefrom; and a plurality of rotatable cutting elements, wherein the plurality of rotatable cutting elements have differing cutting surface geometries based on their radial position along the plurality of blades.
 17. The cutting tool of claim 16, wherein the plurality of rotatable cutting elements are located in the nose and shoulder region of the blades.
 18. The cutting tool of claim 17, wherein the rotatable cutting elements in the nose have a convex cutting surface geometry while the rotatable cutting elements in the shoulder have a concave cutting surface geometry.
 19. The cutting tool of claim 17, wherein the rotatable cutting elements in the nose have a concave cutting surface geometry while the rotatable cutting elements in the shoulder have a convex cutting surface geometry.
 20. The cutting tool of claim 17, wherein the plurality of rotatable cutting elements located in the nose region have a thicker substrate layer supporting the cutting surface than the rotatable cutting elements located in the shoulder region. 